ABSTRACT Over 4 million US adults regularly smoke cigars, which causes multiple cancers, including oral, esophageal, pancreatic, laryngeal, and lung cancer. Little cigar and cigarillo (LCC) use among adults has dramatically increased over the last decade because of disparities in taxation between cigarettes and cigars, LCC pack sizes that make cigar products affordable, and regulations, such as flavor bans, that exist for cigarettes but not for cigars. Little research has examined the effectiveness of currently mandated little cigar and cigarillo (LCC) warnings. Research from studies of cigarette warnings suggests that effective LCC warnings should employ images that illustrate negative health effects associated with use and a large warning label prominently displayed on the pack, and that removal of flavor descriptors may improve attention to warning messages. However, research specific to cigarettes cannot adequately inform LCC warning development due to the differences in product characteristics, user demographics and high legal bar for tobacco warnings. No research exists on the potential impact of such changes on LCC use among current LCC users. Our proposed study will fill the existing gaps by understanding which LCC warning characteristics (i.e., content, format, size) are most influential in reducing LCC use, and how an additional LCC policy, the removal of flavor descriptors on packaging, influences the impact of LCC warnings. In Aim 1, we will use existing research and expert review to develop new text and pictorial LCC warnings and test the new warnings using online experiments to identify warnings that adult LCC users perceive as most effective. We will conduct a national, web-based RCT in Aim 2 to examine whether the most effective warnings with images from Aim 1 increase quit intentions among adult LCC users, compared to the currently mandated warnings and a control condition. While our Aim 2 study will assess how improved warnings influence LCC behavioral intentions, our Aim 3 will assess whether flavor descriptors on LCC packages distract from warnings, reducing their impact. In Aim 3, we will conduct an in-person laboratory study among adult LCC users using objective measures of attention (eye tracking), affect (facial electromyography), and arousal (electrodermal activity) to determine how flavor descriptors influence the effectiveness of improved warnings compared to currently mandated warnings. The proposed study will fill critical gaps regarding which characteristics make LCC warning labels most effective and provide needed evidence on how LCC warnings influence LCC behavioral intentions. Our overarching goal is to develop effective LCC warnings that reduce cancer and other health risks.